Pulse width modulation (PWM) has been extensively applied to various electronic devices. For example, PWM controllers are used in switching power supplies to modulate duty cycles or switching frequencies of power switches and thereby modulate output voltages.
Recently, due to energy shortages and the increasing awareness of environmental protection, the energy saving feature of switching power supplies has drawn more and more attention. At the same time, laws and regulations were passed to impose stricter requirements on the power conversion efficiency of switching power supplies at light load and in standby mode. When a switching power supply is working at light load or in standby mode, the switching loss of its power switches accounts for a significant proportion of the overall power consumption. To increase the light loading and standby power conversion efficiency of switching power supplies, some power management integrated circuits (ICs) on the market are designed to lower the switching frequencies of power switches so that switching loss can be significantly reduced. Moreover, switching power supplies, though advantageously more compact than conventional linear power supplies, have another problem that electro-magnetic interference (EMI) caused by the switching elements. Jittering frequency technique is typically used to improve EMI problem in existing power management ICs. Numerous jittering frequency techniques have been proposed, such as U.S. Pat. No. 7,701,305, whose operation principles are not detailed herein.
Switching power supplies have a variety of types. For example, the switching power supply shown in FIG. 1 has a flyback configuration in which a PWM controller 10 switches a power switch M1 according to voltages at the pins COMP and CS to control the output power of the flyback power supply. The power switch M1 is connected in series to the primary side coil of a transformer 12, and a current sense resistor Rcs is serially connected to the power switch M1 to detect the current of the power switch M1 to generate a current sense signal Vcs supplied to the pin CS of the PWM controller 10. A photo-coupler 14 and a shunt regulator 16 establish an isolated feedback circuit, the shunt regulator 16 detects the output voltage V0 of the flyback power supply to control the feedback current Icomp on the pin COMP of the PWM controller 10, and based on the feedback current Icomp, a circuit in the PWM controller 10 generates a feedback voltage Vcomp at the pin COMP. The feedback voltage Vcomp is proportional to the output voltage V0. The flyback power supply shown in FIG. 1 provides the output power
                                                        Po              =                            ⁢                              η                ×                                  Vin                  2                                ×                                                      Ton                    2                                    /                                      (                                          2                      ×                      Lp                      ×                      T                                        )                                                                                                                                          =                                ⁢                                  η                  ×                                      Vin                    2                                    ×                                      Ton                    2                                    ×                                      fs                    /                                          (                                              2                        ×                        Lp                                            )                                                                                  ,                                                          [                  Eq          ⁢                      -                    ⁢          1                ]            where η is the conversion efficiency of the transformer 12, Vin is the primary side input voltage of the transformer 12, Ton is the on-time of the power switch M1, Lp is the magnetizing inductance of the transformer 12, T is the switching period of the power switch M1, and fs is the switching frequency of the power switch M1. If the PWM controller 10 uses a jittering frequency technique, the switching frequency fs will jitter. As shown by the equation Eq-1, jittering of the switching frequency fs causes variation in the output power Po to generate a low-frequency ripple at the output Vo of the flyback power supply, resulting in excessive output ripple.
In order to solve the excessive output ripple problem caused by jittering frequency, U.S. Pat. No. 7,026,851 generates an adjust current proportional to the jittering frequency to adjust the feedback voltage Vcomp. However, the level of the feedback voltage Vcomp at light load is different from that at heavy load; the constant proportional adjust current may lead to overcompensation at light load.